A number of semiconductor device fabrication procedures include processes in which a material is sputtered from a target onto a workpiece such as a semiconductor wafer. Material is sputtered from the target, which is appropriately biased, by the impact of ions created in the vicinity of the target. A certain proportion of the sputtered material may be ionized by a plasma such that the resulting ions can be attracted to the wafer. The wafer is mounted on a support and is usually biased to a DC potential selected to attract the sputtered, ionized material. Typically, the sputtered material is composed of positive ions and the workpiece is negatively biased.
There are several known techniques for exciting a plasma with RF fields including capacitive coupling, inductive coupling and wave heating. In a standard inductively coupled plasma (ICP) generator, RF current passing through a coil induces electromagnetic currents in the plasma. These currents heat the conducting plasma by ohmic heating, so that it is sustained in steady state. As shown in U.S. Pat. No. 4,362,632, for example, current through a coil is supplied by an RF generator coupled to the coil through an impedance matching network, such that the coil acts as the primary winding of a transformer. The plasma acts as a single turn secondary winding of the transformer.
In certain types of deposition chambers, the chamber walls may be formed of a conductive metal such as stainless steel. Because of the conductivity of the chamber walls, the RF coil is placed within the chamber itself because the conducting chamber walls would block or substantially attenuate the electromagnetic energy radiating from the coil. As a result, the coil may be directly exposed to the deposition flux and energetic plasma particles. This is a potential source of contamination of the film deposited on the wafer, and therefore may be undesirable in some applications. To protect the coils, shields can be made from nonconducting ceramic materials. However, some deposition processes involve deposition of conductive materials such as aluminum on the electronic device being fabricated. Because the conductive material will coat the ceramic shield, the shield will become conducting, thus substantially attenuating the penetration of electromagnetic radiation into the plasma.
U.S. Pat. No. 5,346,578 describes a system in which a plasma is created for the performance of various types of wafer processing operations, including etching and chemical vapor deposition in a quartz vessel surrounded by an exterior induction coil. RF energy is transmitted from the coil through the vessel into the chamber to energize the plasma. A reactive gas is introduced into the vessel in order to be ionized by the plasma, the resulting ions being directed to a wafer under the influence of a suitable electric field. It is believed that the apparatus described in this reference is not well suited for the performance of conductive material sputtering processes because sputtered material tends to coat all interior surfaces of a vessel. As the interior surface of the vessel becomes coated with conductive material, the interior surface becomes relatively opaque to the RF energy from the coil. While it is possible to clean the interior surfaces of the vessel after deposition of conductive material, such cleaning introduces additional steps and slows down the manufacturing process.
Published European Patent Application 0607797 describes a device for generating a plasma in order to perform low pressure chemical vapor deposition or reactive ion etching operations. The system includes a processing chamber having, at its top, a planar spiral coil producing an electromagnetic field which will be coupled with a plasma within the processing chamber, the coil itself being isolated from the interior of the chamber by a flat dielectric window. The window is associated with a conductive shield which is positioned between the window and the coil. The purpose of the shield is to prevent dielectric material from being sputtered from the window. The material which is to be ionized in order to be deposited on a wafer or to perform etching is introduced into the chamber in the form of a process gas. As in the case of the apparatus described in U.S. Pat. No. 5,346,578, supra, the surface of the dielectric window which communicates with the interior of the chamber is prone to being coated with deposition material. Therefore, it is believed that this chamber is likewise not well suited to conductive material sputtering processes, because the dielectric window will become coated with the conductive material.